Fertility and reproductive histories of cancer patients, especially of long term survivors of childhood cancer and of men and women who reproduced during cancer therapy, are studied for information on the possible mutagenicity and teratogenicity of cancer treatment and to discover hereditary patterns of cancer. Current phases include intensive analysis of data from interviews and medical records of 2,305 survivors of childhood cancer and their siblings (as 3,299 of controls) to gather information on subsequent morbidity and mortality (especially additional neoplasms), quality of life, fertility and health of offspring. Four percent of cases had subsequent cancer compared to 2% of the controls. In 7,117 offspring, 18 cancers occurred, sometimes in known patterns of single gene traits and cancer family syndromes. A second phase is the completion of analysis of a voluntary registry of pregnancies in women with cancer that, to date, shows little, if any, excess of birth defects, but some excess wastage of pregnancies conceived within 12 months of completing chemotherapy. Finally, the principles of mutation epidemiology, in particular, the use of so-called sentinel phenotypes, have been reviewed.